1. Field of the Invention
The present invention relates to a magnetic resonance imaging technique for obtaining three-dimensional images of an image slice, and more particularly, to a three-dimensional spectroscopic imaging technique which is a hybrid of one-dimensional Hadamard spectroscopic imaging and two-dimensional chemical shift imaging that allows all slices in the field of view to be simultaneously acquired.
2. Description of the Prior Art
As described by Moonen, et al. in an article entitled "Proton Spectroscopic Imaging of Human Brain," J. Magn. Reson., Vol. 98, pp. 556-575 (1992), by Shungu, et al. in an article entitled "Sensitivity and Localization Enhancement in Multinuclear in vivo NMR Spectroscopy by Outer Volume Presaturation," Magn. Reson. Med., Vol. 30, pp. 661-671 (1993), and by Duyn, et al. in an article entitled "Multisection Proton MR Spectroscopic Imaging of the Brain," Radiology, Vol. 188, pp. 277-282 (1993), proton MR studies of organs such as the human brain require suppression of the subcutaneous lipids and bone marrow signals so that they do not contaminate the much smaller brain metabolite signals. This is done most effectively by outer volume suppression (OVS) combined with selective-excitation of the volume of interest (VOI). As described in the for-mentioned Moonen, et al. and Duyn, et al. articles, as well as by Alger, et al. in an article entitled "Absolute Quantitation of Short TE Brain .sup.1 H MR Spectra and Spectroscopic Imaging Data," J. Comput. Assist. Tomogr., Vol. 17, pp. 191-199 (1993), by Posse, et al. in an article entitled "Short Echo Time Proton MR Spectroscopic Imaging," J. Comput. Assist. Tomogr., Vol. 17, pp. 1-14 (1993), and by Duyn, et al. in an article entitled "Fast Proton Spectroscopic Imaging of Human Brain Using Multiple Spin Echoes," Magn. Reson. Med., Vol 30, pp. 409-414 (1993), finer localization within that volume is achieved with two-dimensional chemical shift imaging. As described in the above-mentioned articles, OVS favors a VOI within an axial or paraxial slice so the fat at the rim can easily be saturated by spatially-selective RF pulses. For these pulses to contain the skull's curvature when imaging the brain, the slice must be thin (a few centimeters), ruling out the use of chemical shift imaging in that direction. Therefore, to localize, a one voxel thick slice is excited and two-dimensional chemical shift imaging is performed in its plane. Three-dimensional coverage is achieved by sequentially interleaving N (usually four) single-slices.
In particular, as shown in prior art FIG. 1, the prior art proton (.sup.1 H) NMR spectroscopy (.sup.1 H-MRS) in the human brain, for example, comprises selective excitation of a single, thin slice (in an axial plane) as shown in step A1, followed by localization of a two-dimensional array of voxels within the plane of that slice with two-dimensional chemical shift imaging (CSI) (step B1). This process is then repeated for each slice (steps A2, B2, A3, B3, A4, and B4). This approach is dictated by the need to suppress the undesirable fat signals of skull bone, skin, and various other subcutaneous lipids at the edges of the slice.
On the other hand, if localization of a larger volume is desired, several slices (usually 4, but sometimes more) may be acquired in an interleaved fashion as shown in FIG. 2(a). However, because of the nature of interleaving, each of the N slices is sampled for only a fraction (1/N) of the cycle time required to complete a cycle of sampling all slices and is idle for the rest ((N-1)/N) of the time. Because the acquisition time of each slice is comparable with the transverse relaxation time of protons, P.sub.2 (.about.1 second), if N=4 slices are to be sampled, each slice will be "revisited" for only 1 second of every 4 second cycle, with a resulting relative signal-to-noise ratio (SNR) of 1/N.sup.1/2.
In particular, the problem with slice-interleaving is that because the T.sub.2 s of in vivo .sup.1 H cerebral metabolites are long (Frahm, et al. in an article entitled "Localized Proton NMR Spectroscopy in Different Regions of the Human Brain in vivo. Relaxation Times and Concentration of Cerebral Metabolites," Magn. Reson. Med., Vol. 11, pp. 47-63 (1989), and Toft, et al. in an article entitled "T.sub.1, T.sub.2 and Concentrations of Brain Metabolites in Neonates and Adolescents Estimated with H-1 MR Spectroscopy," J. Magn. Reson. Imag., Vol. 4, pp. 1-5 (1994) found the T.sub.2 s to be 100-500 ms), the free induction decay (FID) from the i.sup.th slice must be acquired for T.sub.acq. (i).apprxeq.1 second. This brings the recycle time, TR, to ##EQU1## seconds, which is inefficient in SNR/unit-time, since it has been shown in the afore-mentioned Frahm, et al. And Toft, et al. articles, as well as in an article by J. S. Waugh entitled "Sensitivity in Fourier Transform NMR Spectroscopy of Slowly Relaxing Systems," J. Mol. Spec., Vol. 35, pp. 298-305 (1970), and in an article by R. R. Ernst entitled "Sensitivity Enhancement in Magnetic Resonance," Adv. in Magn. Reson., Vol. 2, pp. 1-135 (1966), that the T.sub.1 s of .sup.1 H brain metabolites, 1-1.5 seconds, are significantly shorter. Consequently, good voxel SNR in a 4 slice-interleaved experiment requires approximately 45 minutes of acquisition.
It is desired to substantially reduce the signal acquisition time for such spectroscopic imaging so as to decrease the discomfort to the patient without sacrificing SNR. In addition, it is desired that the resulting images exhibit improved localization and improved fat suppression. The present invention has been designed to meet these needs.